[enhancement]: store last selected format in storage

[andyGallagher]

Hi — long time user. I'm looking to see if it's possible to store a user's last played format in storage so that on page refresh the user does not need to reselect their previous format. I exclusively play gen1randombattle so it's always a bit annoying to have to change from the default gen9randombattle every time I visit the client.

I'm very happy to implement this and contribute here, let me know!